Feb. 18, 2011 — University of Virginia researchers and a recent graduate have shown that the NASA Cassini spacecraft detected a very tenuous atmosphere infused with oxygen and carbon dioxide around Saturn's icy moon, Rhea.
This is the first time a spacecraft has directly captured molecules of an oxygen atmosphere, albeit a very thin one, from a world other than Earth.
The researchers' paper on the subject appears in the November issue of the journal Science.
Ben Teolis, who earned a doctorate in engineering physics from U.Va. in 2007 and is now a research scientist for the Southwest Research Institute in San Antonio, was the lead author on the paper. Raul A. Baragiola, Teolis' former Ph.D. advisor, and Robert E. Johnson, both professors of engineering physics and materials science at U.Va.'s School of Engineering and Applied Science, served as co-authors on the paper. Baragiola and Johnson have been members of the Cassini mission for the past two decades.
Interpretation of the discovery of the oxygen atmosphere was possible thanks to Teolis' laboratory work at U.Va. and Johnson's theoretical predictions from 1982. The researchers' findings extend previous discoveries by the Galileo mission around Jupiter's icy moons and suggest similar scenarios in other solar systems being discovered.
The oxygen appears to arise when Saturn's magnetic field rotates over Rhea. Energetic particles trapped in the planet's magnetic field pepper the moon's water-ice surface. They cause chemical reactions that decompose the surface and release oxygen.
The source of the carbon dioxide is less certain, but studies conducted at Baragiola's laboratory indicate that it, too, may be a result of radiation by energetic particles.
Oxygen levels at Rhea's surface is estimated to be about 5 trillion times less dense than what's present on Earth. But the scientists' interpretation of Cassini's discovery shows that surface decomposition could contribute abundant molecules of oxygen, leading to surface densities roughly 100 times greater than the exospheres of either Earth's moon or Mercury. The formation of oxygen and carbon dioxide could possibly drive complex chemistry on the surfaces of many icy bodies in the universe.
"The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe," Teolis said.
Less clear is a connection to extraterrestrial life, since Rhea is too cold and devoid of the liquid water necessary for life as we know it.
Releasing oxygen through surface irradiation could help generate conditions favorable for life at an icy body other than Rhea that has liquid water under its surface, Teolis said. If the oxygen and carbon dioxide from the surface could somehow get transported down to a sub-surface ocean, such as the ocean discovered by the Galileo spacecraft on Jupiter's moon Europa, that would provide a much more hospitable environment for more-complex compounds and life to form. Scientists are keen to investigate whether life is possible on icy moons with an ocean, though they have not yet detected it.
The tenuous atmosphere infused with oxygen and carbon dioxide makes Rhea, Saturn's second-largest moon, unique in the Saturnian system. Titan has a thick nitrogen-methane atmosphere, but very little carbon dioxide and oxygen.
"Rhea is turning out to be much more interesting than we had imagined," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "The Cassini finding highlights the rich diversity of Saturn's moons and gives us clues on how they formed and evolved."
Scientists had suspected Rhea could have a thin atmosphere, or exosphere, with oxygen and carbon dioxide, based on remote observations of Jupiter's icy moons by NASA's Galileo spacecraft and Hubble Space Telescope. Other Cassini observations detected oxygen escaping from icy Saturn ring particles after ultraviolet bombardment. But Cassini was able to detect oxygen and carbon dioxide in the exosphere directly because of how close it flew to Rhea – 101 kilometers, or 63 miles — and its special suite of instruments.
In the new study, scientists combined data from Cassini's ion and neutral mass spectrometer and the Cassini plasma spectrometer during fly-bys on Nov. 26, 2005, Aug. 30, 2007, and March 2, 2010. The ion and neutral mass spectrometer "tasted" peak densities of oxygen of around 50 billion molecules per cubic meter (1 billion molecules per cubic foot). It detected peak densities of carbon dioxide of around 20 billion molecules per cubic meter (about 600 million molecules per cubic foot).
The plasma spectrometer saw clear signatures of flowing streams of positive and negative ions, with masses that corresponded to ions of oxygen and carbon dioxide.
Researchers are still puzzled by the presence of carbon dioxide. It may be the result of "dry ice" trapped from the primordial solar nebula, as is the case with comets, or it may be due to similar irradiation processes operating on the organic molecules trapped in the water ice of Rhea. Baragiola's laboratory has demonstrated that radiation can produce carbon dioxide on ice-covered carbon surfaces. Another idea is that the carbon dioxide could also come from carbon-rich materials deposited by tiny meteors that bombarded Rhea's surface.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The ion and neutral mass spectrometer team and the Cassini plasma spectrometer team are based at Southwest Research Institute, San Antonio.
This story was adapted from a NASA press release.
The full version of this story with accompanying images is here.